1. Field of the Invention
The present invention relates to an organic light-emitting device including an organic light-emitting element which emits light when an electric current is applied to an organic compound layer containing a light-emitting layer which is formed between a pair of electrodes, and a method for producing the same. Particularly, the present invention relates to an organic light-emitting device including an organic light-emitting element which can emit light of a plurality of colors, and a method for producing the same.
2. Description of the Related Art
Conventionally, an electroluminescence (EL) element and a light-emitting diode (LED) have been known as a self-luminous element for use in a display unit or a light source. Among them, an organic light-emitting element (hereinafter may be referred to as organic EL element or the element) including an organic compound material for a light-emitting layer has received attention, because the element can provide a luminance of hundreds to thousands Cd/m2 or higher, even when being driven by a DC voltage of about 10 V or less.
Furthermore, the organic light-emitting element can emit three primary colors of light when being produced from such appropriately-selected organic compound materials as to emit spectra corresponding to red, green and blue, respectively. In addition, a full-color display unit can be relatively easily produced from a plurality of such organic light-emitting elements which are arranged on a substrate.
A structure of a conventional organic light-emitting element will be described with reference to FIG. 12. FIG. 12 is a schematic view showing a general structure of the conventional organic light-emitting element.
The conventional organic light-emitting element has a transparent anode layer 52 formed on a glass substrate 51; a hole transport layer 53 made of an organic material, an EL layer (light-emitting layer) 54 made of an organic material and an electron transport layer 55 sequentially stacked thereon in this order; and further a metal cathode layer 56 provided thereon, as is shown in FIG. 12. In addition, as needed, a hole injection layer is occasionally provided between the transparent anode layer 52 and the hole transport layer 53, and an electron injection layer is further occasionally provided between the metal cathode layer 56 and the electron transport layer 55.
In the organic light-emitting element shown in FIG. 12, the transparent anode layer 52 formed on the glass substrate 51 transmits light. The metal cathode layer 56 is made of aluminum, magnesium, indium, silver or an alloy thereof, and reflects light. A light emitted from the EL layer 54 passes through the transparent anode layer 52 and is emitted to the side of the glass substrate 51. An element structure of emitting light from the substrate side as described above is referred to as a bottom emission type (hereinafter described as a BE type). On the other hand, an organic EL element having a structure of emitting light to the opposite side of the substrate is referred to as a top emission type (hereinafter described as a TE type). In the top-emission-type organic light-emitting element, in general, an electrode formed on the glass substrate has light reflectivity, and an electrode formed on the opposite side of the above electrode across an organic layer has light-transmissive property.
It has been known that an organic light-emitting element having such a structure as shown in FIG. 12 has dependency of the luminance on a film thickness, which means that the luminance varies according to the film thickness of an electron transport layer 55, because an interference effect formed by a light emitted directly to a glass substrate 51 side and a light reflected on a metal cathode layer 56 also varies according to the thickness of the electron transport layer 55. Thus, a technology of prescribing the thickness of the electron transport layer 55 in a BE-type element is disclosed for the purpose of optimally adjusting an optical path difference between an EL layer 54 and a metal cathode layer 56 that is a reflection layer (see Japanese Patent Application Laid-Open No. H04-328295).
A technology is also disclosed which enhances the color purity of a BE-type organic light-emitting element having the same structure as in the case of Japanese Patent Application Laid-Open No. H04-328295, by optimizing not only the thickness of an electron transport layer but also a total thickness of an optical film of a high-refractive transparent electrode such as ITO and an organic multilayered portion (see Japanese Patent Application Laid-Open No. H07-240277). Specifically, the technology sets the total thickness of the optical film of the transparent electrode and the organic multilayered portion so that the total thickness can enhance the strength of the emitted light in a center wavelength λ, which is generated from the organic multilayered portion having a refractive index of 1.6 to 1.8. The above described λ is selected from the wavelengths of 440 to 490 nm, 500 to 550 nm and 600 to 650 nm. The technology described in the Japanese Patent Application Laid-Open No. H07-240277 optimizes the optical film thickness with respect to a predetermined emission wavelength, by varying the thickness of the high-refractive transparent electrode (such as ITO).
On the other hand, a technology is disclosed which is used in a multicolor organic electroluminescence display unit having a light-emitting layers for emitting lights of a plurality of colors (Japanese Patent Application Laid-Open No. 2000-323277). The multicolor organic EL display unit has many organic EL elements having respective light-emitting layers for emitting lights of red, green and blue colors having different emission spectra arranged on a substrate, and therefore the process of varying a thickness of a transparent electrode (such as ITO) for every different color becomes complicated. Then, the technology described in Japanese Patent Application Laid-Open No. 2000-323277 gives each different film thickness corresponding to each emission color to any of functional layers having the same function (for instance, a hole transport layer or an electron transport layer) in an organic compound material layer except the light-emitting layer, in the multicolor organic electroluminescence display unit. Accordingly, the transparent electrode can be set at a fixed film thickness throughout the all organic EL elements, and has no need of being adjusted into a different thickness corresponding to each color by etching. Incidentally, the technologies described in the Japanese Patent Application Laid-Open Nos. H04-328295, H07-240277 and 2000-323277 provide examples of a BE type.
Furthermore, a technology is disclosed which can be applied to a TE type in which a lower electrode formed on a substrate is a reflective electrode and an upper electrode arranged thereon through an organic layer is a transparent electrode (see Japanese Patent Application Laid-Open No. 2004-253389). The technology described in Japanese Patent Application Laid-Open No. 2004-253389 optimizes an optical path length between a light-emitting layer (light-emitting plane if precisely being described) and a reflective lower electrode, so as to be appropriate to each light of red, green and blue.
Specifically, the technology described in the Japanese Patent Application Laid-Open No. 2004-253389 provides a multicolor organic EL display unit having a structure in which light emitted from a white light-emitting layer is separated into each light of red, green and blue, by using a filter. The thicknesses of the transparent electrode and a transparent optical layer are varied so that a distance between the white light-emitting layer and the lower reflective electrode can be optically optimized in accordance with a light of each color.
Japanese Patent Application Laid-Open Nos. 2005-197011, 2005-093399 and 2005-93401 describes a technology relating to an element structure in which an upper electrode is semi-transparent and forms a micro-resonator structure between the upper electrode and a lower reflective electrode. These technologies optically optimize a distance between a light-emitting layer and the lower reflective electrode and a distance between the upper and lower electrodes, by adjusting a thickness of ITO which is a transparent electrode according to an emission color, by stacking and etching the ITO.
As is described in Japanese Patent Application Laid-Open No. H04-328295, emission intensity is distributed in a light-emitting layer so as to be highest on an interface of the light-emitting layer in a hole injection side. Accordingly, all of the above described conventional technologies consider the interface of the light-emitting layer in the hole injection side (normally, the interface between the light-emitting layer and the hole transport layer) as a light-emitting plane, when designing an optical structure.
In addition, an optical path length is defined as the product (nd) of a refractive index (n) of a medium in which light travels and a distance (d) in which the light travels. By the way, an emitted light passes through a film such as an organic layer and a transparent electrode in an organic EL element, so that a term “optical film thickness” is used as well, but such terms as “optical path difference”, “optical film thickness”, “optical length” and “optical path length” are all used in the same meaning.
However, the above described conventional technologies had various problems to be solved.
Technologies described in Japanese Patent Application Laid-Open Nos. H04-328295 and H07-240277 do not sufficiently disclose how the technologies can be applied to each organic light-emitting element, when an organic light-emitting device has many organic light-emitting elements which emit lights of emission colors such as red, green and blue, formed on the same substrate.
A technology described in Japanese Patent Application Laid-Open No. 2000-323277 optimizes an optical length by adjusting a thickness of an organic compound layer except for a light-emitting layer, in order to apply the technology described in Japanese Patent Application Laid-Open No. H07-240277 to an organic light-emitting device having many organic light-emitting elements which emit lights with a plurality of emission colors, because it is difficult to change a thickness of a transparent electrode (ITO) according to each emission color. However, each organic compound layer, such as an electron transport layer and a hole transport layer, for forming the organic light-emitting element has the optimal thickness which is determined by electric properties such as the injectability and transportability of electrons and holes, and electric resistance. Accordingly, various problems appear when the thickness of each organic compound layer for forming the organic light-emitting element is determined for the purpose of optimizing the optical path length. Specifically, when the film thickness determined for optimizing the optical path length is different from the film thickness determined on the basis of electric characteristics, the organic light-emitting device has to sacrifice the electric characteristics, though being preferable when the former thickness matches the latter thickness by chance. When the film thicknesses are set as is described above, the organic light-emitting element of emitting a light of a particular color occasionally results in needing high driving voltage.
A technology described in Japanese Patent Application Laid-Open No. 2004-253389 describes that a transparent electrode and a transparent optical layer can be patterned by a standard photographic method and the film thickness can be changed by stacking the layer. But, it does not disclose anywhere specifically how to stack the layer.
In addition, Japanese Patent Application Laid-Open Nos. 2005-197011, 2005-093399 and 2005-093401 disclose a technology of changing the thickness of a transparent electrode (ITO and IZO) according to each emission color, which has been considered to be difficult in the technology described by Japanese Patent Application Laid-Open No. 2000-323277. However, the technology of changing the thickness of the transparent electrode according to the emission color needs a complicated step like the step of etching an IZO layer using a crystallized ITO as an etching stop, or the step of forming a resist on a position corresponding to each emission color and etching ITO. Particularly, the ITO has transparency, so that it is difficult to confirm an end point of etching by visual inspection. An optical path length is a product obtained by multiplying the thickness of the transparent electrode (ITO) in which light transmits by the refractive index, so that the transparent electrode needs to be formed in a precise film thickness in order to optically design the optical path length, but the conventional technologies had difficulty in forming the transparent electrode in the precise thickness.
In addition, technologies described in Japanese Patent Application Laid-Open Nos. 2000-323277, 2004-253389, 2005-197011, 2005-093399 and 2005-093401 change the thicknesses of a transparent electrode, a transparent optical layer and a hole transport layer formed on a substrate so as to correspond to a light of each emission color. Accordingly, in a process step of forming an upper electrode, a height from the substrate up to the top surface of an organic compound layer is different depending on a position corresponding to each emission color.
In addition, technologies described in Japanese Patent Application Laid-Open Nos. 2004-253389, 2005-197011, 2005-093399 and 2005-093401 disclose a TE-type organic EL element. The TE-type organic EL element has a light-transmissive electrode (a metal thin film, or a transparent electrode made of ITO, normally by using a sputtering method) formed on the whole top surface of an organic layer as an upper electrode. Particularly, the technologies described in Japanese Patent Application Laid-Open Nos. 2005-197011, 2005-93399 and 2005-93401 form an upper electrode made of a metal thin film so as to make the electrode a semi-light-transmissive electrode having partially reflectivity and partially transparency, in order to form a micro-resonator structure. In addition, the technology described in Japanese Patent Application Laid-Open No. 2005-197011 further forms a transparent conductive layer made of ITO on the upper electrode, so as to lower a resistance value. These upper electrodes having a light-transmissive property need to have a high light-transmissive property for efficiently emitting light to the outside and have a sufficient electroconductive property.
However, technologies described in Japanese Patent Application Laid-Open Nos. 2004-253389 and 2005-093401 result in forming a step (difference of film thickness) between positions corresponding to respective emission colors, because in formation of an organic layer including a light-transmissive electrode layer which is an upper electrode, a height from a substrate up to the top surface of the organic layer is different in the positions corresponding to respective emission colors. Accordingly, in consideration that the upper electrode should not cause discontinuities in a part at which the step is formed (coverage property), the upper electrode needed to be formed thick (for instance, about 100 nm or thicker), when formed of a transparent electrode such as ITO. When the upper electrode is a metal thin film, the thickness is limited to 10 nm or smaller in consideration of the light-transmissive property, and it is difficult to decrease the electric resistance, so that the resistance is decreased by electrically conducting the metal thin film to a auxiliary electrode formed on a substrate, in Japanese Patent Application Laid-Open Nos. 2005-093399 and 2005-093401. In this case as well, when there is the step, the metal thin film is easily cut (tends to cause discontinuities at the step), which is unpreferable. Furthermore, when a sealing film is formed on the transparent electrode layer or the organic layer as the technology described in Japanese Patent Application Laid-Open No. 2004-253389, if the surface on which the sealing film is to be formed has unevenness, the sealing film needs to be formed thick so as to secure its moisture barrier characteristics. However, the thick sealing film has not been desirable, because of lowering the light-transmissive property and needing a longer period of time in a production process.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.